1. Field of the Invention
The present invention relates to phase-change optical information recording media and especially to compact disc rewritable CD−RW media and processes for recording information thereon.
2. Description of the Related Art
In compact discs (CDs) and digital versatile discs (DVDs), information is reproduced based on a change in intensity of light reflected from a medium. In read only memory (ROM) optical discs, there is a substrate that has depressions and protrusions on its surface to cause a phase difference of the reflected light, thus interference occurs to induce the change in intensity.
In recordable media, micro domains having different optical properties are formed in a recording layer arranged on a substrate of the media to induce the change in intensity. The recording layer generally comprises, for example, an organic coloring matter in CD recordable (CD−R) and DVD recordable (DVD+R) discs, or a phase-change recording material in CD rewritable (CD−RW) and DVD rewritable (DVD+RW) discs.
In any case, information is recorded on the discs by irradiation of focused light to the vicinity of the recording layer, which changes the state of the micro domains, and the difference in optical properties of such changed micro domains induces phase difference or intensity difference.
The use of a phase change material in the recording layer allows one to form and erase recording marks, since the phase change between a crystalline phase and an amorphous phase used in recording is reversible. The phase change between the crystalline phase and the amorphous phase can be controlled based on thermal hysteresis including quenching (rapid cooling) and annealing (slow cooling) of the material, and information can be recorded and erased by modulation of intensity of an irradiated light beam, and recording apparatus using the phase change can be produced at low cost. In addition, the recorded information can be reproduced in a read-only apparatus (player), and the phase change recording systems have been widely employed.
Increasing demands have been made on optical discs to have larger capacity and to operate at higher speed with an increasing capacity of electronic information and with an increasing speed of information processing. Recording media having a larger density are the most effective means to satisfy such demands. To increase the density of media, for example, an optical system for use in recording and reproducing information should be changed to have an increased numerical aperture NA or a shorter wavelength, or a modulation system should be changed. Such change can be seen in, for example, the transition from CD to DVD, the capacity of which is much higher than CD. However, conventional CD reproducing apparatus cannot reproduce or read information on such high-density DVD media. The incompatibility in reproduction must be avoided in commercially distributed media. If emphasis is put on compatibility, increasing the speed of operation becomes the biggest challenge.
It is believed that rewritable optical information recording media using a phase-change material are difficult to have a higher recording and/or reproducing speed as compared with write once media using a coloring matter on which information can be recorded only once. To form recording marks on rewritable optical discs at higher speed, a light beam with a higher recording power may be irradiated as in media using a coloring matter. At high speed, however, the recording marks once formed cannot be erased in the media using the phase-change material. This is because scanning at higher speed cannot create “annealing conditions” necessary for forming a crystalline phase in which the recording marks are erased.
Accordingly, increase in scanning speed of phase-change rewritable optical discs is not so much as in optical discs using a coloring matter. For example, CD−R media now available can be recorded at 40-speed (scanning speed of 48 m/s, channel bit rate of 1.7 Gbps), but CD−RW media can be recorded at 10-speed (scanning speed of 12 m/s, channel bit rate of 432 Mbps).
Patent documents which are of public knowledge prior to the filing of the present application are as follows.
Japanese Patent Application Laid-Open (JP-A) No. 2000-313170 discloses an optical information recording medium using, as a material for a recording layer, ((SbxTe1−x)yGe1−y)zM1−z, wherein x, y and z satisfy the following condition: 0.7≦x≦0.9, 0.8≦y<1, and 0.88≦z<1, and M represents In and/or Ga. However, the compositional ratios as specified in the publication are in wide ranges, and the examples described therein shows only a recording layer where M is In and does not show data for verifying the advantages or effects of the use of Ga as M. The publication also does not describe the necessity of Ga and great differences between Ga and In, in contrast to the- present invention. In addition, the publication does never refer to ensuring the storage reliability and improving direct overwriting properties at higher scanning speed of 20 m/s or higher, improvement of which is one of the objects of the present invention.
JP-A No. 2001-56958 discloses an optical information recording medium having a recording layer mainly comprising GeSbTe and further comprising a metal element selected from a wide variety of metal elements and is capable of overwriting at high speed. In Example 16 of the disclosure, an optical information recording medium using Ga0.06Ge0.06Sb0.68Te0.22 is disclosed. However, the term “high speed” as used in the publication is at highest 10 m/s as specified in, for example, its claim 31. The publication does not teach the improvement. in overwriting properties at a scanning speed of 20 m/s or higher as in the present invention, and the alloy composition described in Example 16 is out of the ranges specified in the present invention. In addition, it neither discloses nor indicate the effectiveness of Ga, in contrast to the present invention.
JP-A No. 2001-236690 and Japanese Patent (JP-B) No. 3255051 (JP-A No. 10-172179) each disclose an optical information recording medium having a recording layer mainly comprising SbTe and further comprising an element selected from a wide variety of elements. However, they lack the concrete description on GaGeSbTe alloys and neither disclose nor indicate the direct overwriting properties at a scanning speed of 20 m/s or higher, improvement of which is one of the objects of the present invention, and the effectiveness of Ga which is pointed out by the present invention.
JP-B No. 2629749 (JP-A No. 01-138634) discloses an optical information recording medium having a recording layer mainly comprising GaGeSbTe. However, the recording layer mainly comprises a GeTe alloy or intermetallic compound and thereby has clearly different compositional ranges and properties from the materials mainly comprising a Sb—Te eutectic alloy and further comprising trace amounts of metal elements as in the present invention.
Accordingly, an object of the present invention is to provide an optical information recording medium, especially a CD−RW medium, that can undergo direct overwriting at high speed and to provide a process for recording information thereon.